1. Field of the Invention
This invention relates to a polyorganosiloxane composition containing a storage stabilizer with a specific structure. More particularly, this invention relates to a polyorganosiloxane composition which exhibits excellent long-term storage stability at room temperature and is rapidly cured by heating through addition type reactions.
2. Description of the Prior Art
The prior art teaches polyorganosiloxane compositions comprising a vinyl group-containing polyorganosiloxane, a polyorganohydrogensiloxane and a platinum catalyst as principal components. These compositions, in combination with a storage stabilizer, are cured by heating, which accelerates the addition type curing reaction. Specific examples of such prior art include U.S. Pat. Nos. 3,383,356, 3,445,420 and 3,461,185 and Japanese patent publications Nos. 51-28119 [76-28119], 53-35983 [78-35983]and 54-3774[79-3774]. These prior art compositions can be stored for long periods of time, even after all the components have been mixed, and can be cured only by heating.
Storage stabilizers disclosed in the prior art include organic and organosilicon compounds containing at least one carbon-carbon double bond or carbon-carbon triple bond per molecule. Typical examples of such prior art are U.S. Pat. No. 3,383,356, which discloses ethylene tetrachloride and U.S. Pat. No. 3,445,420, which discloses acetylenic alcohols, such as 3-methyl-1-butyne-3-ol, in addition to silanes and siloxanes containing at least one --C.tbd.C-- radical per molecule. Specific organosilicon stabilizers disclosed in this patent include phenyl tris (2-propynyloxy)silane, phenylmethyl bis(1-methyl-2-propynloxy) silane and sym tetramethyldiethynyldisiloxane.
The activity of all the aforementioned prior art stabilizers is less than satisfactory. Even when these stabilizers are used, the viscosity of the addition-curable polyorganosiloxane composition gradually increases during storage. The use of a large quantity of storage stabilizer in order to completely suppress this increase in viscosity will result in inadequate compatibility and separation of the stabilizer, a requirement for relatively high curing temperatures, or consumption of a large portion of the Si-H groups in the organohydrogenpolysiloxane by reaction with the storage stabilizer, with resultant unsatisfactory curing.
In view of the foregoing drawbacks of prior art compositions, these compositions are usually made commercially available in the form of two liquids to which a small amount of storage stabilizer is added in order to obtain some working time after combining the liquids. The storage stabilizer used in these compositions must suppress the increase in viscosity for a certain period of time after mixing and must be inactivated in the curing process so the composition will cure rapidly. However, this type of storage stabilizer, represented by 3-methyl-1-butynl-3-ol, has a high vapor pressure and is removed in the reduced-pressure defoaming step which is typically employed after the two liquids have been mixed with each other. The effects of the stabilizer are therefore not exhibited and the composition does not have a useful pot life. On the other hand, when an acetylene-type compound containing a larger number of carbons is employed in order to reduce the vapor pressure, the miscibility with the siloxane is poor and curing will be uneven.